I believe it is for all good purposes an apo lens (others will know for sure). I have one and it is outstanding. Excellent contrast, very sharp - it holds up well against current lenses, IMO. But I don't use it much. It fits very well with my M3 and its .92 viewfinder.

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Michael

I would like to manage to prevent people from ever seeing how a picture of mine has been done. What can it possibly matter? What I want is that the only thing emanating from my pictures should be emotion. - Pablo Picasso

It is an amazing lens, especially taking into consideration that is was first produced in 1965! I bought one a few years ago and I like it a lot. I use it much more often than the Elmart 135mm (II) that I also have. The Tele Elmar handles a lot easier and is amazingly sharp, and I like its color rendition a lot.
And as an extra, I can use the lens head on the Bellows II on Visoflex.
Lex

You may try it by taking a photo of fine dark structures with much light in the background. (Tree brances without leaves against the open sky for example). With a "normal - non "apo" lens - you will see "fringing" or violet/green/orange rims on the black structures: chromatic aberrations. With very high resolution and contrast the risk of high chromatic aberrations rise - the 50mm Summilux asph sometimes shows rather high chromatc aberrations in certain situations.

I recently tried out different lenses in the 90mm-range. The 4/90 Elmar with three lenses, which has rather high resolution and contrast showed some chromatic aberrations. The 90mm Summicron Apo-Aspherical - which is supposed to be especially corrected for these situations - showed certainly less, though still some. The 1.5/85 mm Summarex - more than sixty years old - showed none at all - with resolution and contrast rather high at f/8 but still behind the other two lenses.